> ## Documentation Index
> Fetch the complete documentation index at: https://docs.prefect.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Concurrency events

> Events emitted when concurrency limits are created, updated, deleted, and when slots are acquired and released.

Concurrency events cover two categories: CRUD lifecycle events emitted when
concurrency limits are created, updated, or deleted, and slot events emitted
when concurrency slots are acquired and released. For more on concurrency
limits, see [Global concurrency limits](/v3/concepts/global-concurrency-limits).

## Concurrency limit lifecycle events

### `prefect.concurrency-limit.created`

Emitted when a new concurrency limit is created.

#### Resource

| Label                   | Description                        |
| ----------------------- | ---------------------------------- |
| `prefect.resource.id`   | `prefect.concurrency-limit.{uuid}` |
| `prefect.resource.name` | Concurrency limit name             |

#### Related resources

This event has no related resources.

#### Payload

For global (v2) concurrency limits:

| Field                   | Type    | Description                                |
| ----------------------- | ------- | ------------------------------------------ |
| `name`                  | string  | Concurrency limit name                     |
| `limit`                 | integer | Maximum number of concurrent slots         |
| `active`                | boolean | Whether the limit is currently active      |
| `slot_decay_per_second` | float   | Rate at which slots are released over time |

For tag-based (v1) concurrency limits:

| Field               | Type    | Description                       |
| ------------------- | ------- | --------------------------------- |
| `tag`               | string  | Tag this limit applies to         |
| `concurrency_limit` | integer | Maximum number of concurrent runs |

### `prefect.concurrency-limit.updated`

Emitted when a concurrency limit's configuration is changed.

#### Resource

Same as [`prefect.concurrency-limit.created`](#prefectconcurrency-limitcreated).

#### Related resources

This event has no related resources.

#### Payload

Same as [`prefect.concurrency-limit.created`](#prefectconcurrency-limitcreated).

### `prefect.concurrency-limit.deleted`

Emitted when a concurrency limit is deleted.

#### Resource

Same as [`prefect.concurrency-limit.created`](#prefectconcurrency-limitcreated).

#### Related resources

This event has no related resources.

#### Payload

Same as [`prefect.concurrency-limit.created`](#prefectconcurrency-limitcreated).

## Slot events

### `prefect.concurrency-limit.acquired`

Emitted when concurrency slots are acquired for a named limit.

#### Resource

| Label                   | Description                              |
| ----------------------- | ---------------------------------------- |
| `prefect.resource.id`   | `prefect.concurrency-limit.{uuid}`       |
| `prefect.resource.name` | Concurrency limit name                   |
| `slots-acquired`        | Number of slots acquired in this request |
| `limit`                 | Maximum number of slots for this limit   |

#### Related resources

| Resource ID pattern                | Role                | When present                                                                                     |
| ---------------------------------- | ------------------- | ------------------------------------------------------------------------------------------------ |
| `prefect.concurrency-limit.{uuid}` | `concurrency-limit` | One entry per other limit in the same acquisition batch (when acquiring multiple limits at once) |

#### Payload

This event has no payload.

### `prefect.concurrency-limit.released`

Emitted when concurrency slots are released for a named limit. Uses the `follows`
field to link back to the corresponding `acquired` event.

#### Resource

Same as [`prefect.concurrency-limit.acquired`](#prefectconcurrency-limitacquired).

#### Related resources

Same as [`prefect.concurrency-limit.acquired`](#prefectconcurrency-limitacquired).

#### Payload

This event has no payload.

## Legacy v1 concurrency events

<Note>
  These events are emitted by the legacy tag-based concurrency limit system. For
  new concurrency limits, use global concurrency limits which emit the events
  documented above.
</Note>

### `prefect.concurrency-limit.v1.acquired`

Emitted when a legacy tag-based concurrency slot is acquired.

#### Resource

| Label                   | Description                           |
| ----------------------- | ------------------------------------- |
| `prefect.resource.id`   | `prefect.concurrency-limit.v1.{uuid}` |
| `prefect.resource.name` | Tag name                              |
| `limit`                 | Maximum number of slots               |
| `task_run_id`           | ID of the task run acquiring the slot |

#### Related resources

| Resource ID pattern                   | Role                | When present                                            |
| ------------------------------------- | ------------------- | ------------------------------------------------------- |
| `prefect.concurrency-limit.v1.{uuid}` | `concurrency-limit` | One entry per other limit in the same acquisition batch |

#### Payload

This event has no payload.

### `prefect.concurrency-limit.v1.released`

Emitted when a legacy tag-based concurrency slot is released.

#### Resource

Same as [`prefect.concurrency-limit.v1.acquired`](#prefectconcurrency-limitv1acquired).

#### Related resources

Same as [`prefect.concurrency-limit.v1.acquired`](#prefectconcurrency-limitv1acquired).

#### Payload

This event has no payload.